Add two functions to the EnvironmentModifications object to help
users sanitize environment variables in their package definitions:
* deprioritize_system_paths: this keeps system paths in the
environment variable but moves them to the end.
* prune_duplicate_paths: remove any duplicate paths from the
variable
This includes testing for the new functions as well as for
(previously-untested) old convenience functions for environment
variable manipulation.
This also adds special handling for bash functions so they
will be defined when the exported environment file is sourced.
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